Diffuse optical tomography (DOT) imaging can be used as a medical imaging modality where near-infrared (NIR) photons are used to probe tissue. NIR photons are injected into the tissue of interest, for example by focusing a non-invasive NIR laser onto the tissue surface. The NIR photons interact with the underlying physiology. The interactions result in absorption or scattering events, i.e., NIR photons are either annihilated or redirected from their incoming direction, respectively. Scattered photons may eventually escape the tissue of interest. Information gathered from these exiting photons can be used to reconstruct the spatial distribution of physiologically relevant parameters such as tissue absorption and scattering coefficients, concentration of oxygenated and deoxygenated hemoglobin, and concentration of fluorescent and bioluminescent probes. Thus, DOT imaging can be used as tool for monitoring and detecting physiological processes and pathologies.
An application of DOT imaging can be the diagnosis of rheumatoid arthritis (RA) through imaging of finger joints. As compared to other anatomical features, fingers are relatively small, and so the intensity of light transmitted through the fingers can be relatively high, thereby allowing for simple signal detection. Additionally, the scattering properties of the synovial fluid inside the joints changes as a result of the onset of RA. Thus, the optical contrast between healthy joints and joints with RA can be used in the DOT diagnosis of RA.
DOT data acquisition of a structure such as a finger can take on the order of 15 minutes, for example. However, the computational time required to obtain the optical properties of finger joints remains considerably long (e.g., on the order of 3 hours or more per finger). Indeed, computation time can be much longer than 3 hours, depending on the numerical accuracy of the light models employed. Computation times on the order of 12 hours per finger are not unheard of. Moreover, the processing of the DOT data can require significant computational resources (e.g., on the order of 6 GB of RAM or more per finger) that severely limit the practical use of this modality.
A primary factor affecting computation time is the selection of algorithms employed in image reconstruction. In particular, the equation of radiative transfer (ERT) is typically used for modeling light propagation in finger joints because it is the most accurate deterministic model of light propagation in small volumes. Although they are accurate, the ERT-based reconstruction algorithms are also highly complex and require substantial computational resources, thereby increasing the computation time and system requirements necessary to perform such an analysis.